The ordinary street lamp, e.g. high pressure sodium lamp, is constructed by mounting a reflection shade behind the bulb. The reflection shade is a convex shade reflecting the unordered light. The light coming out of this kind of bulbs cannot focus, not only the light shinning out, but also the light reflected from the shade disperse. Some of the dispersing light shin in people's eyes directly and cause vertigo, this is so-called light pollution.
LED street lamp has good chroma, and needs no maintenance with a long service life. Although the LED bulbs focus well, they cannot mount vertical to the earth in reality, because this will leads to small radiation area. However, if the LED bulbs are mounted aslant to the earth, it will cause light pollution just like ordinary street lamps and limit popularization of LED street lamps.